The last few days have been rather frustrating in terms of skiing in large parts of the Alps. T-shirt temperatures in the valleys, or rather T-shirt removal temperatures on many climbing rocks, rapidly dwindling snow and increasingly brownish piste residues are depressing the ski touring motivation. Why is it not really freezing at night, even though the weather is somehow "nice"? Is it going to get cold again and what happens next?
Saharan dust
The brownish-yellow veil that has been clouding visibility for a few days now and has settled over the landscape like a bad Instagram filter is desert dust from the Sahara. It was brought by the strong southerly current that has been affecting us since last week. Saharan dust in the Alps is not particularly unusual, but it's always somehow interesting. Due to the dust in higher layers of air, there are significantly more condensation nuclei than usual on which water particles can collect. This results in the formation of high veil clouds (cirrus clouds), which are generally not predicted by the models. As a result, it sometimes remains much cloudier and cooler than forecast. On the one hand, clouds and dust dampen the radiation and thus the temperatures, on the other hand they prevent night-time radiation, so that the snow cover can only freeze to a limited extent at night, if at all (? no firn but unattractive slushy snow). If the brownish dust settles on the snow, the snow surface can reflect less radiation than usual and melting processes are promoted by the increased energy input (snow melts like chocolate ice cream in the sun).
Saharan dust not only reaches us from time to time, but is also often transported to the Atlantic with the trade winds, or even to the Caribbean and America. It is estimated that around 40 million tons of dust from the Sahara reach the rainforests in the Amazon region every year. The dust is a good fertilizer and it is assumed that the flora there benefits greatly from the gifts of the Sahara. This afternoon or tomorrow at the latest, the dust should disappear from the air with the onset of precipitation. "Blood rain" - brownish colored rain due to the dust - is expected. (The WeatherBlog sees great potential for apocalyptic headlines here.) If the dust falls out with the rain and settles on cars, for example, you should not wipe it off with a cloth as it will scratch the car (compare: cleaning ski goggles).
Outlook: cooler and wetter
Today (Wednesday), a first push of cooler air from the NW has reached the Alps and the foehn-like weather has been interrupted for the time being, with rain setting in towards the afternoon. Tomorrow (Thursday), a cold front will follow, which will be attached to the Alps and stay there for the time being (main ridge possibly still rather clear on Thursday, cloudy in the north). On Friday, a low pressure system in the south will come to the aid of the cold front and together they will cause widespread, relatively heavy precipitation. Temperatures (and therefore the snow line) will drop significantly. It remains to be seen whether our colleague the oracle will get in touch again.
Further outlook: no longer cool and wet
The wintry weather period should be largely over by Sunday. At the beginning of next week, a mild south-westerly current will return, due to a trough on the European Atlantic coast and a strong high in the Mediterranean region. Warm but rather unstable weather is to be expected. The general weather situation is strongly meridional and will probably only change slowly.
